This invention generally relates to a method of preventing wear, and more particularly to a method of preventing fretting and galling in a metallic coupling, such as but not limited to a polygon coupling.
Polygon shaft couplings are generally known. For example, this type shaft coupling is described in U.S. Pat. No. 4,294,562. Typically, in such couplings, a shaft and a hub have cross sections fitting into each other, and these cross sections deviate from a circular pattern and have shapes that are convex on all sides and essentially elliptical, triangular or quadratic.
In a use of this type polygon coupling in centrifugal compressor design, an impeller of a centrifugal compressor is operatively connected to a centrifugal compressor driving shaft or pinion. In such a design, the compressor impeller includes a polygonally dimensioned male hub portion, and the compressor pinion includes a suitably dimensioned bore which receives the male hub portion of the compressor impeller. Although this type polygon coupling has been used with success in centrifugal compressor design, this polygon coupling is susceptible to galling and fretting. More particularly, galling has been known to occur between the two mating polygon surfaces of the hub portion of the impeller and the interior wall of the polygonally dimensioned bore of the pinion. Additionally, fretting has been known to occur at an annular shoulder region between the pinion and the impeller.
As used herein, galling means a type of adhesive wear which is caused by an imbalance of electrons existing between two mating metal surfaces. In this regard, when material is subject to galling under low stresses, minute junctions form at contacting surfaces and small fragments of the metals become detached with subsequent movement. When material is subject to galling under high stresses, large junctions are formed, or actual seizure may occur between the materials. In a polygon coupling of the type described hereinabove, there generally exists high stresses between the impeller and the pinion. When galling occurs between the impeller and the pinion, the impeller may become difficult to remove from the pinion. In cases when the impeller actually seizes on the pinion, the impeller must be cut therefrom to perform any required maintenance or compressor servicing.
As used herein, fretting means surface damage caused by low amplitude, but high frequency sliding between two contacting surfaces. In the polygon shaft coupling between the impeller and the centrifugal compressor pinion, fretting generally causes the annular shoulder region between the pinion and the impellar to fatigue and to wear prematurely.
The foregoing illustrates limitations known to exist in metallic couplings in general, and in particular, in a metallic polygon coupling having a pinion and an impeller, such as in a centrifugal compressor. Thus, it is apparent that it would be advantageous to provide an alternative directed to overcoming one or more of the limitations set forth above. Accordingly, a suitable alternative is provided including features more fully disclosed hereinafter.